In a mode-locked laser, all of the longitudinal modes which are oscillating within the resonator have a common phase relationship. This common phase relationship is necessary to produce very short pulsed output. Since there is a great interest in performing experiments with very short pulses, much effort has been expended in developing schemes for locking the modes in lasers.
At present, there are four known viable approaches for creating mode-locked pulses. The first approach is active mode-locking and requires the insertion of an optical modulator within the laser cavity. In a second and related approach, a laser cavity is synchronously pumped by a separate laser which is itself actively mode-locked. In both cases, an expensive, high stability RF driver is required.
The third approach found in the prior art is passive mode-locking. In this approach, a saturable absorber is placed in the resonant cavity. This approach is cumbersome since the saturable absorber, usually a dye, must be continuously replaced. Another drawback with the passive mode-locking approach is that the laser is tunable only over a narrow range.
Most recently, a scheme has been developed a scheme to couple a principle resonator with an external resonator having a non-linear material therein. This approach is known as additive pulse mode-locking or APM. This approach does not require any RF driver or saturable absorber. However, it is critically dependant on the operating parameters and is therefore difficult to control. Accordingly, it would be desirable to develop a laser system which can be mode-locked without expensive accessories and in a simple manner.